The present invention relates to an improvement in a low-pressure mercury vapor discharge lamp and a method of fabricating the lamp, in which two inner tubes each having one open end and the other closed end are provided within a sealed or air-right glass lamp envelope with a single filament coil being provided inside of the closed end, and more particularly, it relates to a reduction in manufacturing cost for the lamp of this type.
Recently, various attempts have been made to produce fluorescent lamps provided with bulb bases through which the fluorescent lamps may be mounted on associated sockets to light, i.e., bulb base type fluorescent lamps.
FIG. 1 shows a fluorescent lamp of such a type, in particular, a construction of a low-pressure mercury vapor discharge lamp as disclosed in U.S. Pat. No. 4,199,708. This fluorescent lamp is comprised of a glass lamp envelope 1 sealed to a button-stem 2 to form an air-tight space therein and two U-shaped inner tubes 3, 3' each of which has one end adhered to the button-stem 2 to be closed and the other end opened to the air-tight space (at openings 6, 6'), each inner tube being disposed within the air-tight space. Several Torrs of rare gas and a small amount of mercury are enclosed in the glass lamp envelope 1 and the filament coils 4, 4' are disposed inside of the closed ends of the two inner tubes 3, 3'. Discharge is generated between the two filament coils 4, 4' and ultraviolet rays generated from plasma within the inner tubes 3, 3' are converted into visible lights by phosphor layers 5, 5' coated on inner wall surfaces of the inner tubes 3, 3'. A light-dispersing material layer 9 is applied to an inner surface of the glass lamp envelope 1. A starting circuit is enclosed in a case 7 one end of which is provided with a bulb base 8.
The thus constructed fluorescent lamp is characterized in that since the spaces inside of the inner tubes 3, 3', which are luminescent parts, are in communication with the space inside of the glass lamp envelope 1, a mercury vapor pressure in the inner tubes 3, 3', on which a lamp efficiency depends, is determined by a temperature at a cold spot of the glass lamp envelope 1. As a result, it is avantageous that a high power may be obtained without any reduction in efficiency.
However, in the above-described fluorescent lamp, since a discharge path is elongated to enhance the lamp efficiency, the button-stem 2 is used as its stem. In the case where such a button-stem 2 is used in the fluorescent lamp, the following disadvantages are encountered.
(i) It is very difficult to fabricate the button stem 2 of the large diameter, in which lead wires are embedded.
(ii) Since the inner tubes 3, 3' are attached to the button stem 2 by adhesives, the adhering process is time-consuming.
(iii) Since the inner tubes 3, 3' are held on the button-stem by adhesives, a mechanical strength against a collision or the like is small.
As a result, the fabricating cost is increased and reliability is decreased. Therefore, there have been strong demands to overcome these disadvantages, in mass-production of the fluorescent lamps.